1. Field of the Invention
The present invention relates generally to a method of operating a plurality of components operatively associated with a gas turbine engine.
In the next generation of high performance aircraft, engines will be lighter, more efficient, and faster responding and will require more reliable, fault-tolerant controls and accessories. Since the traditional hydromechanical approaches utilize mature technologies, electrical technologies look promising and are being investigated to meet these increased demands. Advances in power electronics and machine technologies are making electric accessory applications possible. Key enabling technologies supporting these applications include the development of electronic devices such as the power MOSFET (metal oxide semiconductor field effect transistor), the IGBT (insulated gate bipolar transistor), the ceramic capacitor, and the MCT (MOS controlled thyristor).
2. Description of the Prior Art
In the traditional hydromechanical system, controls and accessory components are driven from a gearbox that is mechanically coupled to the gas generator shaft. In the new approach, the gearbox is replaced by electric motors which drive the accessories independently. This approach has the potential of greatly simplifying or even eliminating the engine accessory gearbox and providing greater flexibility in locating the accessories on the engine.
Key components for the successful application of electric controls and accessories include a starter/generator capable of operating at engine compressor speeds, an electric motor driven engine fuel pumping system, and an electric motor driven lubricating oil pumping system. Small lightweight power electronics are essential for this new approach to be a viable alternative to the existing technology. In designing aircraft turbine engines with electric controls and accessories, consideration must be given to which functions can be accomplished with electric drives. Typical functions of small engine accessories that are amenable to electric drive applications are engine starting, fuel pumping, lubricating oil pumping, as already noted, and inlet particle separation, and electric power generation.
For a complete understanding of the invention, it is noteworthy that the term "starter/generator" is used throughout the disclosure to mean a device which operates as a starter at low speeds, that is, below idle, and operates to generate electricity at higher speeds, that is, above idle. The term "idle speed", when referring to gas turbine engines, is taken to mean the minimum operational speed, that is, approximately 50% of the maximum speed of which the engine is capable.
All electric accessories customarily receive control signals from the main engine computer. During normal operation, the engine starter is required for less than a minute for each start attempt and is not used after completion of engine starting. It is generally sized to produce adequate torque to start the engine. Electric power generation is usually initiated after the starter has been turned off. This situation lends itself to a natural integration of the starting and generating function into one electrical machine -- the starter/generator. This combination implies that the starter/generator be connected to the engine compressor shaft either through a gearbox or directly coupled. When directly coupled, a starter generator may be required to have a rotational speed capability in excess of 50,000 rpm, a common speed for small engines.
Modern small motors used for driving the accessories just mentioned are preferably brushless machines which conventionally require their own electronic power supply or motor controller for starting and, thereafter, for regulating its speed as long as the gas turbine engine continues to operate. The development of brushless controllable speed motors has made rapid strides in the last decade. Brushless motor concepts emerged in the 1930's with the advent of vacuum tube power control systems and the need for speed and position control of motors. Patents from that period indicate much activity from that time on. However, apparently the state of the technology did not support the inventions to the point of commercial use of any significance.
It took the availability of modern semiconductors and new magnet materials, together with the requirements of the automation industry to spur the development of the modern brushless servo or controllable speed and position motors. With the rapid expansion of electronic and magnetic technology in the last three decades, great changes have indeed taken place in the area of speed and position motor control. The development did not always follow the most efficient path; instead, in each time period, it would concentrate in areas of greatest need, available components, cost of components and the level of understanding the control technology.
Thus, the electronic development in the 1960's was influenced mainly by the development of transistor circuits and the analog operational amplifier and associated components. The subsequent development of low cost logic components, memory arrays and eventually the microprocessor steered development in more recent years towards control systems which became capable of retaining and processing information, and thus able to handle increasingly more complex calculations.
In the area of power control, the early development of the thyristor (SCR), and around the same time, power transistors provided new methods for controlling electric power in an efficient manner, on both the high and low end of the power and frequency spectrum. These devices enabled designers to make amplifiers and inverters for servo control purposes, with the greatest impact on high-power applications using thyristors (notably the current source inverter). In the lower-power end (below 5 kW), the transistor became an efficient power conversion device especially when the pulse width modulation and pulse frequency modulation techniques became practical.
Concurrently, with the development of electronic power conversion device, the development of brushless controllable motors proceeded along several different lines. The availability, in time, of several different permanent magnet materials (such as ALNICO varieties, ferrites, and more recently rare earth magnet materials) offers opportunities for design of a variety of servo type motors for applications ranging from digital tape transport systems to machine tool axis drives and, indeed, now to drives for accessories for gas turbine engines. A particular benefit of the rare earth magnetic materials resides in the high flux levels thereby provided per unit weight. This, in turn, results in a significant reduction of the rotor moment of inertia which can be achieved in comparison with other competitive equivalent motors.
It was earlier mentioned that a.c. motors are preferred for operating the accessories of a gas turbine engine. While the speed of a conventional d.c. motor can be readily regulated, a most desirable feature, inherent in d.c. motor design are the mechanical commutator and the brushes, which wear out and require continuous maintenance. Such problems are eliminated, however, in the a.c. motor which has brushless construction and, therefore, is highly reliable and requires very low maintenance. Additionally, the a.c. motor can operate at higher speeds than its d.c. counterpart and hence has high power to volume and weight ratio. Furthermore, due to its inherently simple structure, it can be built ruggedly and economically. In short, a.c. motors are preferred for their simplicity, lightness, and maintainability.
A.c. motor operation is based on the concept of the rotating magnetic field. This rotating magnetic field is generated by the stator windings carrying polyphase currents. The speed of rotation of the magnetic field is directly proportional to the frequency of the polyphase currents, referred to as synchronous speed. For example, for a 60 Hz, two pole motor, the synchronous speed is 3600 rpm. The most efficient way to change speed is to vary the input frequency. This function is performed by means of an electronic motor controller of the modern type, also as noted earlier.
As stated earlier, the next generation of gas turbine engines will employ a starter/generator that first starts the engine, then serves to power electrically driven accessories such as the oil pump, fuel pump, cooler fans, and the like. Since the small motors used for driving the accessories are conventionally brushless machines, they require electronic power supplies, usually one for each motor, for starting and, thereafter, for speed control. Indeed, the fuel pump motor customarily utilizes a dual power supply for fail-safe operation. It was with recognition of the excessive weight, complexity, and expense of providing and operating such a system having a plurality of motors, each with its own power supply or motor controller, that the present invention has been conceived and is now reduced to practice.